Device and method for fabricating diffractive gratings

ABSTRACT

There is provided a grating fabrication device and method to form gratings on a semiconductor substrate. The substrate is loaded into a reactor filled with an etchant solution, and an array of parallel light of interference light with different periods is projected onto the substrate to etch the portion of the substrate that is exposed to the light via an oxidation-reduction reaction. At the same time, the inclination angle of the substrate is selectively varied to obtain the different grating periods.

This application is a Divisional Application of U.S. Ser. No.09/827,874, filed Apr. 6, 2001 now U.S. Pat. No. 6,547,919.

CLAIM OF PRIORITY

This application claims priority to an application entitled, “Device andMethod for Fabricating Diffractive Gratings,” filed in the KoreanIndustrial Property Office on Jun. 29, 2000 and there duly assignedSerial No. 2000-36372.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an apparatus and method forfabricating diffractive gratings, and particularly, to an apparatus andmethod for fabricating semiconductor material comprising gratings.

2. Description of the Related Art

Optical gratings have a variety of uses including frequency selection,optical feedback-type devices and wavelength dispersion. Easy, rapid,and reliable methods of making gratings of high quality with closespacing and high sensitivity are highly desirable in fabricating LDS.

Generally, diffractive gratings are fabricated using a photolithographictechnique ordinarily used. A photoresist layer, which is resistant tothe etching action, is first formed on a semiconductor substrate andirradiated by interference pattern light. After developing thephotoresist, the semiconductor substrate is etched to a predetermineddepth, except for the portion covered with the photoresist layer.Thereafter, the photoresist layer is removed from the substrate afteretching.

FIG. 1 illustrates a conventional grating fabrication device 10, whichincludes a light source 11, a beam splitter 12, and reflective mirrors13 and 14 for generating an interference pattern on a semiconductormaterial. The reflective mirrors 13 and 14 are arranged in such a waythat the beams split by the beam splitter 12 can be focused on thesurface of substrate 15.

The formation of diffractive gratings on a semiconductor substrate 15using the conventional fabrication device 10 is explained hereinafter.

The surface of semiconductor substrate 15 is first cleaned, then aphotoresist layer 16 about 500 Angstroms (Å) thick or higher is formedon the semiconductor substrate 15. Then, the resultant structure isexposed to the light beams for a predetermined time. The interferencepattern formed by the illumination of light is made by developing thephotoresist. Here, the ratio of light exposed portions and unexposedportions covered with the photoresist layer 16 is about one-to-one.Interference pattern on the substrate is formed by etching with apredetermined solution (e.g., HBr-family etchant solution). Finally, thephotoresist layer 16 is removed using a photoresist stropper, so that apattern is obtained.

The conventional fabrication method, as described in the precedingparagraphs, has some drawbacks in that: (1) the manufacturing process isvery complicated; (2) the frequent possibility of the thin photoresistlayer 16 falling off the substrate 15 makes it difficult to achievereproducibility and product yields; (3) the plasma ashing must becontrolled by repeatedly measuring the light reflectance of thesubstrate 15 and continued until the intended light reflectance isachieved to form both the residual photoresist layer portions and theexposed portions at the intended ratio; (4) the measurement errorsassociated with the light reflectance measuring operation tend todiminish the accuracy; and, (5) the lengthy time (about 7 hours)associated in the fabrication process deteriorates the productivity andimpedes the desired mass production.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a deviceand process for fabricating diffractive gratings in a rapid andsimplified way, thus increasing the product outputs as well as theproduct reproducibility.

To achieve the above object, there is provided a process for fabricatingdiffractive gratings which includes the steps of: providing asemiconductor substrate in a reactor, exposing an interference lightpattern onto the semiconductor substrate; and, supplying a positive ionetchant solution in the reactor to etch the substrate via anoxidation-reduction process.

Preferably, the interference pattern exposurer device includes a lightsource, a beam splitter for splitting the light emitted from the lightsource into different paths, and light path changing means for focusingthe split light beams at different angles onto the surface of thesubstrate.

Preferably, the reactor includes a support member for supporting thesubstrate, and the support member is selectively rotatable with respectto the reactor.

Preferably, interference patterns are formed on the substrate withdifferent periods by changing the inclination of the substrate withrespect to the incident light during the light irradiation step.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates a conventional grating fabrication device;

FIG. 2 illustrates a simplified diagram of the grating fabricationdevice according to the present invention; and,

FIG. 3 illustrates energy bands for different types of semiconductorsubstrates and oxidation-reduction potentials according to the types ofthe positive ion etchant solutions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be describedhereinbelow with reference to the accompanying drawings. For the purposeof clarity, well-known functions or constructions are not described indetail as they would obscure the invention in unnecessary detail.

Referring to FIG. 2, in carrying out the invention according to theembodiment of the present invention, there is provided a gratingfabricating device comprising an interference pattern exposurer 20,which includes a light source 21, a shutter 25, a first lens 26, asecond lens 27, a beam splitter 22, and reflective mirrors 23 and 24,and a reactor 30.

The first lens 26 is a convex lens, which functions as a light enlargerto enlarge or expand the light beam emitted at a predetermined anglefrom the light source 21. The shutter 25 is provided in front of thelight source 21 to transmit/block the beam generated from the lightsource 21. The second lens 27 is a convex lens positioned away from thefirst lens 26 by a predetermined distance and functions as a parallellight conversion means by reshaping the light output by the first lens26 into parallel light beam.

The beam splitter 22 receives the light output from the second lens 27,then splits the received beam into two beams traveling in differentpaths. These two split beams are reflected by the first reflectivemirror 23 and the second reflective mirror 24, respectively, and therespective reflected beams are merged to be focused on at a desiredposition, as shown in FIG. 2. The first and second reflective mirrors 23and 24 are used as light path changing means, and configured in such away that the reflected beams merging from different angles onto thesurface of the substrate 31 is focused at a desired location. It shouldbe noted that although one embodiment of light exposing mechanism isdescribed, there are other related devices known to those skilled inthis art that can be used for projecting light beams into a reactor.

In the embodiment of the present invention, the reactor 30 includes asupport member 33 mounted to the bottom of the reactor 30 at one end,and a holder 32 is rotatably fixed to the support member 33 at the otherend. The holder 32 is adaptable to receive a semiconductor substrate 31to be mounted thereon, and operable to rotate so that the incident angleof the interference light beams being exposed thereto can be varied. Asshown in FIG. 2, the period of an interference pattern incident on thesubstrate 31 is defined by an angle θ at which the interference patternlight reaches the substrate 31. Therefore, the interference patternperiod can be controlled by adjusting the rotation angle α of the holder32 and the incident angle θ.

The substrate 31 displaced along the holder 32 is disposed in thereactor 30, then etchant solution 34 is poured into the reactor 30 foretching the substrate 31 by an oxidation-reduction reaction.Accordingly, the present invention forms gratings on the substrate 31 inwhich the grating is made by imaging an array of parallel lines of light(formed by varying the rotation angle α of the holder 32) on the surfaceand then etched via an oxidation-reduction reaction. Here, the presenceof light is realized from the deflected light beams from the mirrors 23and 24 to induce etching. The etchant solution 34 is in an electrolesspositive ion state and reduced by receiving electrons from electron-holepairs that are generated by the light illuminated on the substrate 31.Here, the superfluous holes oxidize the irradiated portions of thesubstrate 31.

The principle of photo-etching, as described in the precedingparagraphs, occurs by oxidizing the portions of the semiconductorsubstrate 31 exposed to light with the band-gap energy of the substrate31 or higher. The electron-hole pairs are generated on the exposedportions of the substrate 31, and the etchant solution 34 is reduced byreceiving the electrons. Then, the substrate portions oxidized by theholes are etched to a predetermined thickness.

Accordingly, selection of the etchant solution 34 is varied according tothe band-gap energy of the semiconductor device 31. Preferably, theoxidation-reduction reaction of the etchant solution 34 is within theband-gap energy range of the semiconductor substrate 31, and theoxidation energy distribution of the etchant solution 34 issubstantially similar to the conductance band of the semiconductorsubstrate 31. For example, if the substrate 31 is an InP substrate, theetchant solution may be Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ or H⁺/H₂.

Now, a method of fabricating diffractive gratings utilizing the deviceshown in FIG. 2 is explained hereinafter.

First, any impurities are removed from the semiconductor substrate 31 byan ordinarily used cleaning process. The substrate 31 is loaded on theholder 32 that is provided in the reactor 30. Etchant solution 34 ispoured into the reactor. However, it should be noted that the substrate31 may be placed in the reactor 30 after the etchant solution 34 isfilled, or vice versa.

Thereafter, the substrate 31 is exposed to the interference lightpattern having the band-gap energy of the substrate 31 or higher,emitted from the interference pattern exposurer 20 for a predeterminedtime. The exposure time is controlled by the shutter 25.

The presence of light generated by light source 21 and eventuallydeflected by the respective mirrors 23 and 24 will induce etching. Here,portions of the substrate 31 exposed to the light, defining theinterference pattern, are etched through reaction with the etchantsolution 34, whereas the unexposed portions of the substrate 31 do notreact with the etchant solution 34. As a result, diffractive gratingsare formed on the substrate 31. Meanwhile, diffractive gratings can beformed on the substrate 31 in different periods by changing the incidentangle θ of the interference pattern light projected onto the substrate31. To achieve this, the rotation angle α of the holder 32 isselectively adjusted to form diffractive gratings with differentperiods.

FIG. 3 is a graph showing the energy bands for different types ofsemiconductor substrates, the oxidation-reduction potentials, and thepositive ion solutions.

Referring to FIG. 3, since a Ce⁴⁺/Ce³⁺ solution has a highoxidation-reduction potential, it can inject holes into all of GaP,GaAs, and InP. Thus, this etchant solution can etch GaP, GaAs, and InP,regardless of the presence of light irradiation. In the case of an InPsubstrate, an etchant solution Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ may be used tosatisfy the photo-etching condition.

Accordingly, the inventive method relying on the above-describedphoto-etching process can be applied to a variety of semiconductors byappropriately selecting the etchant solution 34 which enablesphoto-etching to occur on the substrate 31. In particular, the inventivemethod is very effective to form diffractive gratings on the InPsubstrate such as a Distributed FeedBack (DFB) semiconductor laser. Inan actual simulation of fabrication grating using the inventive process,it took about two hours to fabricate semiconductor materials comprisingdiffractive gratings.

While the invention has been shown and described with reference to acertain preferred embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and the scope of the inventionas defined by the appended claims.

What is claimed is:
 1. A method for fabricating diffractive gratingsusing a fabrication device, which includes a reactor filled with apredetermined etchant solution and a light generating device forilluminating interference light beams at a predetermined angle to saidreactor, the method comprising the steps of: loading a substrate intosaid reactor so that said substrate is submerged in said etchantsolution; illuminating said substrate with said interference light beamsgenerated by said light generating device; and, etching the portion ofsaid substrate submerged in said etchant solution that is exposed tosaid interference light beams generated by said light generating device;and selectively changing the inclination of said substrate to formdifferent gratings having different periods with respect to an incidentlight angle of said interference light beam.
 2. The method of claim 1,wherein said substrate is an InP substrate and said etchant solution isFe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻.
 3. A method for fabricating diffractive gratingsusing a fabrication device, which includes a reactor filled with apredetermined etchant solution and a light generating device forilluminating interference light beams at a predetermined angle to saidreactor, the method comprising the steps of: loading a substrate intosaid reactor so that said substrate is submerged in said etchantsolution; illuminating said substrate with said interference light beamsgenerated by said light generating device; and, etching the portion ofsaid substrate submerged in said etchant solution that is exposed tosaid interference light beams generated by said light generating device,wherein said substrate is an InP substrate and said etchant solution isH⁺/H₂.
 4. A method for fabricating gratings, the method comprising thesteps of: loading a substrate into a reactor, said substrate disposed ina horizontal orientation with respect to said reactor; pouring apredetermined etchant solution into said reactor to submerge saidsubstrate in said etchant solution; illuminating said substrate with atleast one parallel line of an interference light beam generated by alight generating device; and, etching the portion of said substratesubmerged in said etchant solution that is simultaneously exposed to theinterference pattern light generated by said light generating device;and selectively changing the inclination of said substrate with respectto an incident light angle of said interference light beam.
 5. Themethod of claim 4, wherein said substrate is an InP substrate and saidetchant solution is Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻.
 6. A method for fabricatinggratings, the method comprising the steps of: loading a substrate into areactor, said substrate disposed in a horizontal orientation withrespect to said reactor; pouring a predetermined etchant solution intosaid reactor to submerge said substrate in said etchant solution;illuminating said substrate with at least one parallel line of aninterference light beam generated by a light generating device; and,etching the portion of said substrate submerged in said etchant solutionthat is simultaneously exposed to the interference pattern lightgenerated by said light generating device, wherein said substrate is anInP substrate and said etchant solution is H⁺/H₂.